Reliable air/fuel ignition in a liquid-fueled, direct-injection (DI) engine depends on adequate vaporization of fuel in the engine's combustion chambers. At cold start, however, and especially when the engine temperature is low, adequate vaporization of the fuel may be difficult to achieve. Further, the temperatures where vaporization becomes an issue may increase with decreasing volatility of the fuel (e.g., regular gasoline, premium gasoline, summer gasoline, alcohol-based fuels, diesel fuel, in order of decreasing volatility). To compensate for inadequate vaporization of liquid fuels at low engine temperatures, a fuel-injection control unit may be configured to adjust the rate of fuel-injection in response to engine temperature, and to fuel the engine's combustion chambers at an increased initial rate when the engine temperature is low. Effectively, the stratagem is to flood the intake port or combustion chamber with liquid fuel, expecting only a portion of the liquid fuel to evaporate. However, various disadvantages are associated with overfueling a DI engine during cold start conditions.
A first problem relates to torque control during the run-up period, viz., the period after the engine starts but before a stable idle is achieved. If, as a result of cold-start overfueling, a significant amount of unvaporized fuel accumulates in the combustion chambers of an engine, an unwanted surge of torque may occur during run-up, when the fuel finally vaporizes and is combusted. Some engine systems are configured to intentionally run up the engine speed to clear out excess fuel left over from the start up, but this strategy is inelegant and degrades fuel economy.
A second problem relates to emissions-control performance. During cold start, a DI engine system may emit the same quantity of hydrocarbon as it does over several hours of sustained operation. Excessive hydrocarbon emissions may result from exhaust-system catalysts being underheated, from deliberate enrichment of the pre-ignition air/fuel mixture to enhance ignition reliability (as discussed above) and from unreliable ignition, i.e., misfire, occurring during the first few expansion strokes. If misfire occur at this time, multiple and/or extended cranking attempts may be necessary to start the engine, further worsening emissions-control performance.
A third problem relates to the ability of the engine's high-pressure pump to provide the necessary initial rate of fueling to all of the combustion chambers of the engine. Depending on conditions, initial injection rates required for cold starting may be great enough to overwhelm the capacity of (i.e., to outstrip) the high-pressure pump, especially if the pump is engine-driven and has a relatively small capacity—as in a gasoline direct-injection (GDI) engine, for example.
To address at least some of these and other problems associated with cold-start overfueling in DI engine systems, various countermeasures have been devised. A countermeasure directed to the fuel-delivery problem in GDI engines has been to pump up the fuel rail while the engine is cranking, but to deliver no fuel to the combustion chambers until the fuel rail is fully pressurized. Once the fuel rail is fully pressurized, the injection sequence begins and ignition is attempted. This countermeasure may suffer from a number of drawbacks, however. First, cranking periods are necessarily extended because ignition is delayed until the fuel rail is fully pressurized. Second, the rapid decrease in fuel-rail pressure when the fuel is finally delivered may cause injection-mass control difficulties, resulting in difficult or failed starting. Third, the accumulated fuel-rail pressure may be exhausted before the first firing occurs, should firing occur at all. As a result, multiple and/or extended cranking attempts may be necessary to start the engine.
A countermeasure directed to the torque-control problem described above is to leave some combustion chambers unfueled during cold start at low engine temperatures. In this manner, the accumulation of unvaporized fuel in the combustion chambers of the engine is reduced, thereby limiting the surge of torque that may occur during run-up, when the accumulated fuel vaporizes and is combusted. This strategy may also help to limit overheating of exhaust-stream catalysts during the run-up, which could occur if an excessive amount of uncombusted fuel were to enter the exhaust stream. A potential disadvantage of this countermeasure is that some combustion chambers in an engine may be prone to misfire due to degradation of one or more components—fuel injectors, valve seals, spark plugs, for example. If a combustion chamber prone to misfire is among those included for fueling in a starting sequence in which only a limited number of combustion chambers are fueled, the engine may not develop adequate torque to start. Thus, a potentially useful additional countermeasure that might otherwise be modified to address the fuel-delivery and emissions-control problems described above is compromised by misfire during cranking.
To address the connection between misfire and hydrocarbon emissions, various approaches to detect misfire in a combustion chamber have been disclosed. For example, misfire may be detected based on the angular velocity of a crankshaft measured at selected crank angles, as described in U.S. Pat. Nos. 5,357,790 and 6,658,346. Misfire detection has been used in a number of ways to improve engine performance; U.S. Pat. No. 5,870,986, for example, describes a system in which fuel injection timing is adjusted based on whether a misfire in a combustion chamber is detected. However, none of the approaches cited above address the effect on emissions-control performance of misfire in the first fueled combustion chamber during start-up.
The inventors herein have recognized the issues discussed above and have provided a series of approaches to address at least some of them. Therefore, in one embodiment, a method for starting an engine of a motor vehicle under varying temperature conditions is provided, the engine having a plurality of combustion chambers and a pump for pressurizing fuel for delivery to the combustion chambers. The method comprises, during a first, higher-temperature, starting condition, directly injecting fuel into all of the combustion chambers during at least an initial fueled cycle of the engine, and spark igniting the fuel to increase the rotation speed of the engine. In this context, the initial fueled cycle comprises two rotations of a crankshaft of the engine during which at least some fuel is injected for a first time since the engine was brought from rest. The method further comprises, during a second, lower-temperature, starting condition, directly injecting fuel into less than all of the combustion chambers during at least the initial fueled cycle of the engine, and spark igniting the fuel to increase a rotation speed of the engine. This action may prevent the engine's high-pressure pump from being outstripped during cold-start conditions at low engine temperatures. Also, it may allow subsequently fueled cylinders to start at a higher engine speed and lower manifold air pressure than otherwise possible, thereby further reducing the need for overfueling.
In another embodiment, a method for starting an engine of a motor vehicle is provided, the engine having an intake manifold, an intake throttle controlling admission of air into the intake manifold, and a plurality of combustion chambers communicating with the intake manifold. This method comprises providing a reduced pressure of air in the intake manifold prior to delivering fuel or spark to the engine, the reduced pressure of air responsive to a temperature of the engine. The method further comprises delivering fuel to one or more of the plurality of combustion chambers in an amount based on the reduced pressure of air, and delivering spark to the one or more combustion chambers to start the engine. Other embodiments disclosed herein provide more particular methods, and engine-system configurations in which the various methods may be enacted. In this manner, a GDI engine system may achieve a more reliable cold start at low engine temperatures and with little or no added hardware cost. Further, the cranking time for low-temperature starting may be reduced by not having to build up excessive fuel pressure prior to ignition. And finally, hydrocarbon emissions during low-temperature starts may be reduced by fueling a reduced number of combustion chambers, whilst passing over those combustion chambers that are prone to misfire.
Injecting fuel into low pressure air may result in markedly faster evaporation of liquid fuel than injecting into atmospheric or higher pressure air. Further, by controlling the absolute manifold air pressure, one can make every start occur under more similar conditions regardless of elevation or barometric pressure. Providing consistency over a wide range of cold-start conditions may further reduce the engineering and testing required to find a workable fueling formula and/or protocol.
In short, starting on less than all cylinders reduces the overall need for overfueling during cold start at low engine temperatures. Reduced or controlled manifold air pressure starts have a double effect of reducing the fueling requirement while increasing the fraction of fuel evaporated. Enacted separately or together, both of these actions may have further advantageous effects.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.